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FOR a detailed understanding of the mechanical and physical behavior of metals it is necessary to have a precise knowledge of the absolute and relative energies of interfaces, such as surfaces, grain boundaries, and stacking faults. In recent years a number of techniques for evaluating the stacking fault energy, 7, of metals have been developed, based on electron microscope observations of defects containing stacking faults. Methods using measurements of extended dislocation configurations such as nodes and extrinsic-intrinsic fault pairs are confined to low fault energy materials, since it is only for these materials that the dislocation extension is large enough to measure. Results have therefore been restricted to silver of the pure metals, and alloys for which 7 < 20 erg per sq cm. All the analyses so far used to relate the shape of a node to a value of 7 have involved considerable approximations and the assumption, difficult to test, that the configuration is in equilibrium. Extrinsic-intrinsic fault pairs only give relative energy values and, like nodes, are subject to possible solute segregation effects. Another method b a s e d on the s i z e s of s t a c k i n g fault t e t r a h e d r a and F r a n k loops o b s e r v e d in d e f o r m e d m a t e r i a l s has b e e n applied by L o r e t t o e t a l . ~ to m e t a l s of s l i g h t l y h i g h e r fault e n e r g y , but m o r e r e c e n t r e s u l t s o b t a i n e d by B e e s t o n e t a l . 2 show that the t e t r a h e d r a method g i v e s v a l u e s which a r e c o n s i s t e n t l y lower than the t r u e v a l u e and not in v e r y good a g r e e m e n t with those o b t a i n e d f r o m node o b s e r v a t i o n s . None of the above m e t h o d s is a p p l i c a b l e e i t h e r to high s t a c k i n g fault e n e r g y m a t e r i a l s o r to a wide r a n g e of fault e n e r g i e s . However, a r e c e n t m e t h o d involving the study of the k i n e t i c s of faulted loop a n n e a l i n g , i n i t i a l l y p o i n t e d out by Edington and S m a l l m a n a and developed by Dobson and S m a l l m a n , 4 is p a r t i c u l a r l y a p p l i c a b l e to i n t e r m e d i ate and high ~ m a t e r i a l s and p o t e n t i a l l y useful over a wide r a n g e of fault e n e r g i e s . In the p r e s e n t p a p e r the k i n e t i c s of d i s l o c a t i o n loop a n n e a l i n g have been e x a m i n e d for high, i n t e r m e d i a t e , R. E. SMALLMANis Feeney Professor of Metallurgy and Head of the Department of Physical Mctallurgyand Science of Materials, The University of Birmingham,Birmingham, England. P. S. DOBSON is Lecturcr, Department of Physical Metallurgy and Science of Materials, Thc University of Birmingham. This manuscript is based on a talk prcsented at the symposium on The Measurement of Stacking Fault Energy, sponsored by the IMDTMS PhysicaI MetallurgyCommittee, Pittsburgh, Pa., May 14-16, 1969. METALLURGICALTRANSACTIONS

and low stacking fault energy metals to illustrate the general usefulness of q